1. Field of the Invention
The present invention relates to a charged particle beam generator, a charged particle irradiation system, a method for operating the charged particle beam generator and a method for operating the charged particle irradiation system. The invention more particularly relates to a charged particle beam generator provided with a circular accelerator and an injection linear accelerator, a method for operating the charged particle beam generator provided with the circular accelerator and the injection linear accelerator, a charged particle irradiation system that is provided with an irradiation device for irradiating a tumor (target to be irradiated) such as a tumor with a charged particle beam extracted from a charged particle beam generator and cures the tumor through the irradiation with the charged particle beam, and a method for operating the charged particle irradiation system provided with the irradiation device.
2. Description of the Related Art
A linear accelerator for injection is provided for a circular accelerator such as a synchrotron and located on the upstream side of the circular accelerator. The linear accelerator accelerates charged particles generated by an ion source until the charged particles have predetermined energy appropriate for injection into the circular accelerator. Then, the circular accelerator which has received the charged particle further accelerates the charged particles until the charged particles have higher energy. The thus-accelerated charged particles are used for particle beam therapy with which a patient is cured by irradiating the tumor (such as cancers) of the patient with charged particle beams.
A radio frequency voltage is conventionally used to accelerate the injection linear accelerator provided for the circular accelerator for particle beam therapy. The injection linear accelerator includes an RF power supply device for generating radio frequency voltages. Also, an ion source used for generating charged particles needs a radio frequency voltage for ion generation and includes another RF power supply device for generating radio frequency voltages. With this, the operation cycle of the linear accelerator is determined by that of the RF power supply device. The minimum operation cycle of the linear accelerator is 0.5 seconds or a frequency of 2 Hz (Non-Patent Document 1 (Kenji SAWADA, et al. “Design, Manufacture, and Performance Test of the Injector for Hyogo Hadrontherapy Center” Proc. of The 12th Symposium on Accelerator Science and Technology, Wako Japan 1999), 367 page) or 0.2 seconds or a frequency of 5 Hz (Non-Patent Document 2 (M. Maier et. al “Commissioning of the Linac for the Heidelberg Heavy Ion Center Therapy Center (HIT)” Proc. of Particle Accelerator Conference 2007), page 2734).
The operation cycle of the linear accelerator is fixed, or the minimum operation cycle of the linear accelerator is limited. This is due to the fact that when the operation cycles of the RF power supply devices are increased to a value that is three or four times the fixed cycle or minimum cycle, the operations or radiofrequency characteristics of the RF power supply devices depart from stationary operations, become unstable and affect characteristics of the beam. Further, this is due to the fact that when the operation cycles of the RF power supply devices are reduced to a fraction of the fixed cycle or minimum cycle, thermal loads or the like of the RF power supply devices or thermal loads or the like of radiofrequency devices are increased and the operations of the devices become unstable and affect the characteristics of the beam. Thermal loads resulting from reduction in the operation cycles may bring about a failure in devices due to their heat. Thus, for the purpose of device protection, the time (operation cycle) it takes from operation to operation is made longer. That is, the minimum operation cycle needs to be limited.
There is a method of supplying a charged particle beam from a beam injection device at desired timings when RF power supplies in a linear accelerator provided for acceleration and an ion source operates in a fixed cycle or a substantially fixed cycle. Such a method is known in Non-Patent Document 3 (Akira Takagi OHO' 96 High Energy Accelerator Seminar, High-intensity proton accelerator of Hadron Project 1996, pages I-17, 18). Specifically Non-Patent Document 3 describes a method for adding a delay time (sufficiently smaller than the operation cycle) to one of the two timings such that charged particles are prevented from being accelerated when they are not necessary, and for accelerating the charged particle beams only when they are necessary while the frequency voltage for acceleration is matched with the frequency voltage for the ion source without the addition of the delay time.
When the charged particle beam accelerated by the circular accelerator is used for particle beam therapy, the heart rate or respiration of the patient may move his or her tumor from a set position. To cope with this, the circular accelerator is controlled such that the charged particle beam is extracted only when the tumor is located in place (See Japanese Patent No. 3518270). However, when charged particle beams are to be injected into the circular accelerator at a timing as described in Japanese Patent No. 3518270, the operation cycle of the beam injection device (injection accelerator) is fixed or the minimum operation cycle of the beam injection device is limited in some cases. In such a case, a waiting time, i.e., a time period corresponding to one operation cycle of the beam injection device is required at maximum based on the time when the charged particle beams are to be injected. As a result, it is considered that the circular accelerator cannot be operated as desired and irradiation time is increased by a time period corresponding to the waiting time; accordingly a burden on the patient may be further added.
In addition, Japanese Patent Nos. 3518270 and 2833602 describe an irradiation method in which when the charged particle beam accelerated by the circular accelerator is used for particle beam therapy, tumor irradiation is performed as follows. The tumor is divided into layers in a depth direction and the charged particle beam is scanned across each of the layers in alignment with the tumor shape. After the irradiation of the layer is completed, the tumor is irradiated while the energy of the charged particle beam extracted from the circular accelerator is changed. As shown in FIG. 8 of Japanese Patent No. 2833602, when the circular accelerator is to change a layer to be irradiated, a beam extraction signal is transmitted to the injection accelerator for acceleration of charged particles. However, the operation cycle of the injection accelerator is fixed or the minimum operation cycle of the injection accelerator is limited in some cases. In such a case, a waiting time, i.e., a time period corresponding to one operation cycle of the beam injection device is required at maximum based on the time when the charged particle beams are to be injected. As a result, it is considered that the circular accelerator cannot be operated as desired and irradiation time is increased by a time period corresponding to the waiting time; accordingly a burden on the patient may be further added.